1. Field of the Invention
The invention relates to radiology systems using a radiographic film and, more particularly to a method that can be used to determine and calibrate the lumination (i.e. the quantity of light received multiplied by the exposure time) received by the radiographic film obtain a given blackening of said film.
2. Description of the Prior Art
A radiology system essentially comprises an X-ray tube and a detector of such radiation, between which the object to be examined, for example a part of a patient's body, is interposed. The image receiver which is, for example, a film/screen couple, gives an image of the object after an appropriate exposure time and following development of the film. To enable the image of the object to be exploited as efficiently as possible, the different dots that constitute it should have sufficient contrast with respect to one another, namely, the blackening of the radiographic film should be appropriate from one X-ray shot to the next one, despite the possible differences in opacity of the radiographed object.
The blackening of the film is related to the quantity of energy of the radiation incident to the film/screen couple, namely, the product of the intensity of the radiation to which the radiographic film is subjected, or "film" dose rate, by the time during which the film is exposed to this radiation. Consequently, to obtain a constant blackening of the film from one radiography to another, there is a known way of making measurements, during the examination, of the incident energy on the film by means of a detection cell, generally placed before the receiver, that is sensitive to X-radiation and gives a current proportional to the "film" dose rate. This current is integrated, from the start of the exposure, in an integrator circuit that increases during the exposure. This increasing value is compared, during the exposure time, with a fixed reference value, established beforehand as a function of the characteristics of the film. The end of the exposure time is determined by the instant at which the comparison indicates that the value representing the incident energy on the film is equal to the reference value.
Should the radiographic film be directly subjected to X-radiation, and should the variation in the exposure times from one examination to another be small enough, a constant blackening of the film is obtained from one exposure to the next one, independently of the duration of the exposure time S and provided that the product of the exposure time S and the dose rate F is constant, i.e. the value resulting from the integration should remain constant. This is true only if the characteristics of the film obey the law of reciprocity which indicates that the optical density of the film is proportional to the product F.times.S and if the response of the film is independent of the quality of the incident X-ray beam. This law of reciprocity is no longer met when the variation in the exposure times is great.
Besides, should the radiographic film be associated with an intensifying screen, the blackening of the film depends on the quality of the spectrum. For, the response of the screen depends on the energy distribution of the spectrum of the radiation received, which means that it is sensitive to the hardening of the spectrum and to the change in voltage of the X-ray tube. The deviation from the law of reciprocity, which varies according to the type of film, represents the relative variation of the lumination needed to obtain a constant optical density when the exposure time S varies while the spectrum of the X-radiation is constant. This is expressed by the fact that, to obtain a same optical density of the film, the lumination should be, for example 1 for an exposure time S=0.1 second, 1.3 for S=1 second and 2 for S=4 seconds.
This deviation from the law of reciprocity is due to the phenomenon known as the Schwarzschild effect. This effect is described notably in the work by Pierre GLAFKIDES, CHIMIE ET PHYSIQUE PHOTOGRAPHIQUES (Photographic Chemistry and Physics), 4th edition, pages 234 to 238, PUBLICATIONS PHOTO-CINEMA Paul MONTEL.
To take account of this deviation from the law of reciprocity, various approaches have been proposed, and one of them has been described in the French patent No. 2 584 504. This patent proposes the comparison of the integrated value of the signal given by the detection cell with a reference value that varies during the exposure according to a determined relationship. More precisely, from the start of each exposure time, an additional value is added to the difference between the value of the integrated signal and the reference value. This additional value increases as a function of time according to a previously determined relationship, for example an exponential relationship.
This previously determined relationship, whether it is exponential or otherwise, takes account of the deviation from the law of reciprocity only imperfectly. In particular, it does not take account of the variations in the luminous intensity effectively received by the film. Furthermore, this correction does not take account of the effects of other phenomena such as the hardening of the X-radiation due to the thickness of the object crossed or the modification of the spectrum due to the voltage of the X-ray tube. Furthermore, in this method, the detection cell is placed before the image receiver. An object of the present invention, therefore, is to implement a method that enables the estimation and calibration of the lumination received by a radiographic film by means of measurements made by the detector placed behind the radiographic film.